Method for recording information on optical recording medium, information recorder, and optical recording medium

ABSTRACT

It is an object of the present invention to provide an information recording method for recording information in a data rewritable type optical recording medium having a plurality of information recording layers, which can form recording marks having good shapes. In the information recording method according to the present invention, information is recorded in an optical recording medium  10  having at least a stacked L0 layer  20  and L1 layer  30  by projecting a laser beam thereonto whose power is modulated between a plurality of powers including at least a recording power (Pw) and an erasing power (Pe) via a light incidence plane  13   a . When information is recorded, λ/NA is set to be equal to or shorter than 700 nm, where λ is a wavelength of the laser beam and NA is a numerical aperture (NA) of an objective lens, and a ratio (Pe/Pw) of the recording power and the erasing power when information is to be recorded in the L0 layer  20  is set to be smaller than that when information is to be recorded in the L1 layer  30.

BACKGROUND OF THE INVENTION

The present invention relates to a method of recording information in anoptical recording medium, and particularly to a method of recordinginformation in a data rewritable type optical recording medium having aplurality of information recording layers. Further, the presentinvention relates to an information recording apparatus for recordinginformation in an optical recording medium, and particularly to aninformation recording apparatus for recording information in a datarewritable optical recording medium having a plurality of informationrecording layers. Furthermore, the present invention relates to anoptical recording medium, and particularly to a data rewritable opticalrecording medium.

DESCRIPTION OF THE PRIOR ART

Optical recording media typified by the CD and the DVD have been widelyused as recording media for recording digital data. The recordingcapacity demanded of such optical recording media has increased year byyear, and various proposals have been made to achieve this. One of theseproposals is a technique that uses a two-layer structure for theinformation recording layers contained in the optical recording media,which has found practical application in the DVD-Video and DVD-ROMformats which are read-only optical storage media. With such read-onlyoptical recording media, pre-pits formed on the substrate surface becomethe information recording layer, and such substrates have a laminatedstructure with an intervening intermediate layer.

In addition, in recent years, proposals have been made for opticalrecording media with a two-layer structure for the information recordinglayer to be used also as an optical recording medium in which data canbe rewritten (data rewritable type optical recording medium) (SeeJapanese Patent Application Laid Open NO. 2001-273638). Such a datarewritable type optical recording medium has a structure in which arecording film and dielectric films between which they are sandwichedform an information recording layer, and these information recordinglayers are laminated.

A phase change material is generally used for forming a recording filmof a data rewritable type optical recording medium and data are recordedutilizing the difference in the reflection coefficients between the casewhere the recording film is in a crystal phase and the case where it isin an amorphous phase. More specifically, in an unrecorded state,substantially the entire surface of the recording film is in a crystalphase and when data are recorded, the phase of a predetermined region ofthe recording film is changed to the amorphous phase to form a recordingpit. The phase of the phase change material in the crystal phase can bechanged to the amorphous phase by heating the phase change material to atemperature equal to or higher than the melting point thereof andquickly cooling it. On the other hand, the phase change material in theamorphous phase can be crystallized by heating the phase change materialto a temperature equal to or higher than the crystallization temperaturethereof and gradually cooling it.

Such heating and cooling can be performed by adjusting the power(output) of a laser beam. In other words, it is possible not only torecord data in an unrecorded recording film but also to directlyoverwrite (direct-overwrite) a recording mark already formed in a regionof the recording film with a different recording mark by modulating theintensity of the laser beam. Generally, the power of the laser beam ismodulated in accordance with a pulse waveform having an amplitudebetween a recording power (Pw) and a bottom power (Pb) in order to heatthe recording film to a temperature equal to or higher than the meltingpoint thereof and the power of the laser beam is set to the bottom power(Pb) in order to quickly cool the recording film. Further, in order toheat the recording film to a temperature equal to or higher than thecrystallization temperature thereof and gradually cool it, the power ofa laser beam is set to an erasing power (Pe). In this case, the erasingpower (Pe) is set to a level at which the recording film is heated to atemperature equal to or higher than the crystallization temperaturethereof and lower than the melting point thereof, thereby performingso-called solid phase erasing.

Here, in a data rewritable type optical recording medium having twoinformation recording layers, since data are recorded or reproduced byfocusing a laser beam onto one of the information recording layers, inthe case of recording data in or reproducing data from the informationrecording layer farther from the light incidence plane (hereinafterreferred to as an “L1 layer”), a laser beam is projected thereonto viathe information recording layer closer to the light incidence plane(hereinafter referred to as an “L0 layer”). Therefore, since it isnecessary for the L0 layer to have a sufficiently high lighttransmittance, it is general for the L0 layer to include no reflectivefilm or even if the L0 layer includes a reflective film, the thicknessof the reflective film is set to be very thin.

Since the L0 layer thus includes no reflective film or even if the L0layer includes a reflective film, the thickness of the reflective filmis set to be very thin in a data rewritable type optical recordingmedium having two information recording layers, the heat radiationcharacteristic of the L0 layer is lower than that of the L1 layerincluding a sufficiently thick reflective film and, therefore,re-crystallization of the phase change material tends to occur. Morespecifically, since metal is generally used as the material for forminga reflective film, heat generated in the L1 layer by irradiation with alaser beam can be quickly radiated through the reflective film havinghigh thermal conductivity but since the L0 layer includes no reflectivefilm or only a very thin reflective film, heat generated in the L0 layerby irradiation with a laser beam cannot be quickly radiated. A recordingmark (an amorphous region) formed in the L0 layer is therefore deformedand a good signal cannot be reproduced.

Particularly, in recent years, attempts have been made to record largequantities of data by setting the quotient (λ/NA) of the wavelength λ ofthe laser beam used for recording and/or reproducing divided by thenumerical aperture (NA) of the objective lens used to focus the laserbeam to be equal to or shorter than 700 nm, for example, by setting thenumerical aperture NA to 0.7 or greater, e.g. roughly 0.85, and alsoshortening the wavelength λ of the laser beam to about 200 to 450 nm inorder to make the focused spot diameter of the laser beam smaller andincrease the recording density. In such a system that records and/orreproduces data using a laser beam of short wavelength converged by anobjective lens having a high NA, the above mentioned influence ofthermal interference becomes great in the L0 layer and the phase changematerial tends to be re-crystallized.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aninformation recording method for recording information in a datarewritable type optical recording medium having a plurality ofinformation recording layers, which can form recording marks having goodshapes.

Further, another object of the present invention is to provide aninformation recording apparatus for recording information in a datarewritable type optical recording medium having a plurality ofinformation recording layers, which can form recording marks having goodshapes.

Moreover, a further object of the present invention is to provide a datarewritable type optical recording medium having a plurality ofinformation recording layers, in which recording marks having goodshapes can be formed.

The above object of the present invention can be accomplished by aninformation recording method for recording information in a datarewritable type optical recording medium having at least stacked firstand second information recording layers by projecting a laser beamthereonto whose power is modulated between a plurality of powersincluding at least a recording power (Pw) and an erasing power (Pe) viaa light incidence plane, the information recording method comprisingsteps of setting λ/NA to be equal to or shorter than 700 nm where λ is awavelength of the laser beam and NA is a numerical aperture (NA) of anobjective lens, and, when recording information in the optical recordingmedium, setting a ratio (Pe/Pw) of the recording power and the erasingpower when information is to be recorded in the first informationrecording layer to be smaller than that when information is to berecorded in the second information recording layer.

In a preferred aspect of the present invention, the first informationrecording layer is located on the side of the light incidence plane withrespect to the second information recording layer.

In a further preferred aspect of the present invention, information isrecorded by with at least one of a pulse width of a top pulse and apulse width of a multi-pulse when information is to be recorded in thefirst information recording layer set to be narrower than acorresponding pulse width(s) when information is to be recorded in thesecond information recording layer.

In a further preferred aspect of the present invention, information isrecorded with a cooling interval set to be longer when information is tobe recorded in the first information recording layer than wheninformation is to be recorded in the second information recording layer.

In a further preferred aspect of the present invention, the laser beamhas a wavelength of 200 to 450 nm.

The above object of the present invention can be also accomplished by aninformation recording method for recording information in a datarewritable type optical recording medium having at least stacked firstand second information recording layers by projecting a laser beamthereonto whose power is modulated between a plurality of powersincluding at least a recording power (Pw) and an erasing power (Pe) viaa light incidence plane, the information recording method comprising astep of, when recording information in the optical recording medium,setting a ratio (Pe/Pw) of the recording power and the erasing powerwhen information is to be recorded in the first information recordinglayer to be 0.38 to 0.66 times that when information is to be recordedin the second information recording layer.

The above object of the present invention can be also accomplished by aninformation recording apparatus for recording information in a datarewritable type optical recording medium having at least stacked firstand second information recording layers by projecting a laser beamthereonto whose power is modulated between a plurality of powersincluding at least a recording power (Pw) and an erasing power (Pe) viaa light incidence plane, the information recording apparatus beingconstituted so as to set λ/NA to be equal to or shorter than 700 nmwhere λ is a wavelength of the laser beam and NA is a numerical aperture(NA) of an objective lens, and, when recording information in theoptical recording medium, set a ratio (Pe/Pw) of the recording power andthe erasing power when information is to be recorded in the firstinformation recording layer to be smaller than that when information isto be recorded in the second information recording layer.

The above object of the present invention can be also accomplished by anoptical recording medium which has at least stacked first and secondinformation recording layers and in which information can be recorded byprojecting a laser beam thereonto whose power is modulated between aplurality of powers including at least a recording power (Pw) and anerasing power (Pe) via a light incidence plane, the optical recordingmedium comprising setting information required for setting λ/NA to beequal to or shorter than 700 nm where λ is a wavelength of the laserbeam and NA is a numerical aperture (NA) of an objective lens, and, whenrecording information therein, setting a ratio (Pe/Pw) of the recordingpower and the erasing power when information is to be recorded in thefirst information recording layer to be smaller than that wheninformation is to be recorded in the second information recording layer.

In a preferred aspect of the present invention, the optical recordingmedium further comprises a light transmission layer for forming anoptical path of the laser beam and the light transmission layer has athickness of 30 to 200 μm.

According to the present invention, recording marks having good shapescan be formed even when information in any one of the informationrecording layers is directly overwritten.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section illustrating the structure of anoptical recording medium 10 according to a preferred embodiment of thepresent invention.

FIG. 2 is a drawing illustrating a part of a process (a step for forminga substrate 11) for manufacturing an optical recording medium 10.

FIG. 3 is a drawing illustrating a part of a process (a step for formingan L1 layer 30) for manufacturing an optical recording medium 10.

FIG. 4 is a drawing illustrating a part of a process (a step for forminga transparent intermediate layer 12) for manufacturing an opticalrecording medium 10.

FIG. 5 is a drawing illustrating a part of a process (a step for formingan L0 layer 20) for manufacturing an optical recording medium 10.

FIG. 6 are a set of waveform diagrams showing pulse train patterns usedfor recording data in an L0 recording film 22 and an L1 recording filmwherein FIG. 6(a) shows a case of recording a 2T signal, FIG. 6(b) showsa case of recording a 3T signal, FIG. 6(c) shows a case of recording a4T signal and FIG. 6(d) shows a case of recording one of a 5T signal toan 8T signal.

FIG. 7 is a schematic drawing of the major components of an informationrecording apparatus 50 for recording data in an optical recording medium10.

FIG. 8 is a graph showing a relationship between a ratio (Pe0/Pw0) of arecording power (Pw0) and an erasing power (Pe0), and jitter.

FIG. 9 is a graph showing a relationship between a ratio (Pe1/Pw1) of arecording power (Pw1) and an erasing power (Pe1), and jitter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be explained indetail with reference to the drawings.

FIG. 1 is a schematic cross section illustrating the structure of anoptical recording medium 10 according to a preferred embodiment of thepresent invention.

As shown in FIG. 1, an optical recording medium 10 according to thisembodiment includes a substrate 11, an intermediate layer 12, a lighttransmission layer 13, an L0 layer 20 provided between the intermediatelayer 12 and the light transmission layer 13 and an L1 layer 30 providedbetween the substrate 11 and the intermediate layer 12. The L0 layer 20constitutes an information recording layer far from a light incidenceplane 13 a and is constituted by a first dielectric film 21, an L0recording film 22 and a second dielectric film 23. Further, the L1 layer30 constitutes an information recording layer close to the lightincidence plane 13 a and is constituted by a third dielectric film 31,an L1 recording film 32 and a fourth dielectric film 33. In this manner,the optical recording medium 10 according to this embodiment includestwo information recording layers (the L0 layer 20 and the L1 layer 30).

The substrate 11 is a disc-like substrate having a thickness of about1.1 mm serving as a support for ensuring mechanical strength requiredfor the optical recording medium 10 and grooves 11 a and lands 11 b areformed on the surface thereof. The grooves 11 a and/or lands 11 b serveas a guide track for the laser beam L when data are to be recorded inthe L1 layer 30 or when data are to be reproduced from the L1 layer 30.Although the depth of the groove 11 a is not particularly limited, it ispreferably set to 10 nm to 40 nm and the pitch of the grooves 11 a ispreferably set to 0.2 μm to 0.4 μm. Various materials can be used forforming the, substrate 11 and the substrate 11 can be formed of glass,ceramic, resin or the like. Among these, resin is preferably used forforming the substrate 11 since resin can be easily shaped. Illustrativeexamples of resins suitable for forming the substrate 11 includepolycarbonate resin, olefin resin, acrylic resin, epoxy resin,polystyrene resin, polyethylene resin, polypropylene resin, siliconeresin, fluoropolymers, acrylonitrile butadiene styrene resin, urethaneresin and the like. Among these, polycarbonate resin or olefin resin ismost preferably used for forming the substrate 11 from the viewpoint ofeasy processing, optical characteristics and the like. In thisembodiment, since the laser beam L does not pass through the substrate11, it is unnecessary for the substrate 11 to have a light transmittanceproperty.

The intermediate layer 12 serves to space the L0 layer 20 and the L1layer 30 apart by a sufficient distance and grooves 12 a and lands 12 bare formed on the surface thereof. The grooves 12 a and/or lands 12 bserve as a guide track for the laser beam L when data are to be recordedin the L0 layer 20 or when data are to be reproduced from the L0 layer20. The depth of the groove 12 a and the pitch of the grooves 12 a canbe set to be substantially the same as those of the grooves 11 a formedon the surface of the substrate 11. The depth of the intermediate layer12 is preferably set to be 10 μm to 50 μm. The material for forming theintermediate layer 12 is not particularly limited and an ultraviolet raycurable acrylic resin is preferably used for forming the intermediatelayer 12. It is necessary for the transparent intermediate layer 12 tohave sufficiently high light transmittance since the laser beam L passesthrough the transparent intermediate layer 12 when data are to berecorded in the L1 layer 30 and data recorded in the L1 layer 30 are tobe reproduced.

The light transmission layer 13 forms an optical path of a laser beamand a light incident plane 13 a is constituted by one of the surfacesthereof. The thickness of the light transmission layer 13 is preferablyset to be 30 μm to 200 μm. The material for forming the lighttransmission layer 13 is not particularly limited and, similarly to theintermediate layer 12, an ultraviolet ray curable acrylic resin ispreferably used for forming the light transmission layer 13. Asdescribed above, it is necessary for the light transmission layer 13 tohave sufficiently high light transmittance since the laser beam L passesthrough the transparent intermediate layer 13.

Each of the L0 recording film 22 and the L1 recording film 33 is formedof a phase change material. Utilizing the difference in the reflectioncoefficients between the case where the L0 recording film 22 and the L1recording film 33 are in a crystal phase and the case where they are inan amorphous phase, data are recorded in the L0 recording film 23 andthe L1 recording film 33. The material for forming the L0 recording film22 and the L1 recording film 33 is not particularly limited but it ispreferable to form them using a SbTe system material. As the SbTe systemmaterial, SbTe may be used alone, or InSbTeGe, AgInSbTe, Ag SbTeGe,AgInSbTeGe or the like containing In, Te, Ge, Ag or the like asadditives may be used.

Since the laser beam passes through the L0 recording film 22 when dataare recorded in the L1 layer 30 and data recorded in the L1 layer 30 arereproduced, it is necessary for the L0 layer 20 to have a high lighttransmittance. Therefore, the thickness of the L0 recording film 22 isset to be considerably thinner than that of the L1 recording film 32.Concretely, it is preferable to set the thickness of the L1 recordingfilm 32 to be about 3 to 20 nm and the thickness of the L0 recordingfilm 22 to be 0.3 to 0.8 times that of the L1 recording film 32.

The first dielectric film 21 and the second dielectric film 23 formed soas to sandwich the L0 recording film 22 serve as protective films forthe L0 recording film 22 and the third dielectric film 31 and the fourthdielectric film 33 formed so as to sandwich the L1 recording film 32serve as protective films for the L10 recording film 32. The thicknessof the first dielectric film 21 is preferably set to be 2 to 200 nm, thethickness of the second dielectric film 23 is preferably set to be 2 to200 nm, the thickness of the third dielectric film 31 is preferably setto be 2 to 200 nm and the thickness of the fourth dielectric film 33 ispreferably set to be 2 to 200 nm.

Each of these dielectric films may have a single-layered structure ormay have a multi-layered structure including a plurality of dielectricfilms. The material for forming each of these dielectric films is notparticularly limited but it is preferable to form it of oxide, nitride,sulfide, carbide of Si, Al, Ta and Zn such as SiO₂, Si₃O₄, Al₂O₃, AlN,TaO, ZnS, CeO₂ and the like or a combination thereof.

The reflective film 34 serves to reflect the laser beam entering throughthe light incident plane 13 a so as to emit it from the light incidentplane 13 a and the thickness thereof is preferably set to be 20 to 200nm. The material for forming the reflective film 34 is not particularlylimited but the reflective film 34 is preferably formed of an alloycontaining Ag or Al as a primary component and may be formed of Au, Ptor the like. Further, a moisture proof film may be provided between thereflective film 34 and the substrate 11 in order to prevent thereflective film 34 from being corroded. Materials usable for formingeach of the first dielectric film 21 to the fourth dielectric film 33can be used for forming the moisture proof film. Further, although theL0 layer 20 includes no reflective film, a thin reflective film having athickness of about 3 to 15 nm may be provided in the L0 layer 20. Inthis case, the reflective film can be formed of the same material asused for forming the reflective film 34.

When data are recorded in the thus constituted optical recording medium10, a laser beam having a wavelength of 200 to 450 nm is projected ontothe optical recording medium 10 via the light incidence plane 13 a andthe amount of the laser beam reflected from the optical recording medium10 is detected. As described above, since the L0 recording film 22 andthe L1 recording film 32 are formed of the phase change material and thereflection coefficient in the case where the phase change material is inthe crystal phase and that in the case where it is in the amorphousphase are different from each other, it is possible to judge byprojecting the laser beam via the light incidence plane 13 a, focusingit onto one of the L0 recording film 22 and the L1 recording film 32 anddetecting the amount of the laser beam reflected therefrom whether aregion of the L0 recording film 22 or the L1 recording film 32irradiated with the laser beam is in the crystal phase or the amorphousphase.

When data are to be recorded in the optical recording medium 10, a laserbeam having a wavelength of 200 to 450 nm is projected to be focusedonto one of the L0 recording film 22 and the L1 recording film 32 and inaccordance with data to be recorded therein, a predetermined region ofone of the L0 recording film 22 and the L1 recording film 32 is heatedto a temperature equal to or higher than the melting point thereof andquickly cooled, thereby changing the phase thereof to the amorphousphase or a predetermined region of one of the L0 recording film 22 andthe L1 recording film 32 is heated to a temperature equal to or higherthan the crystallization temperature and gradually cooled, therebychanging the phase thereof to the crystal phase. The region whose phasehas been changed to the amorphous phase is referred to as “a recordingmark” and recorded data are expressed by the length from the startingpoint of the recording mark to the ending point thereof and the lengthfrom the ending point thereof to the starting point of the nextrecording mark. The length of each recording mark and the length betweenrecording marks (edge to edge) are set to one of the lengthscorresponding to 2T through 8T (where T is the clock period) whenadopting the (1,7) RLL modulation scheme, although this is no particularlimitation. A pulse train pattern used for recording data in the L0recording film 22 and a pulse train pattern used for recording data inthe L1 recording film 32 will be described later.

When recording data in or reproducing data from the L1 layer 30, a laserbeam is projected onto the L1 recording film 32 via the L0 layer 20.Therefore, it is necessary for the L0 layer 20 to have a high lighttransmittance and, as pointed out above, the thickness of the L0recording film 22 is set to be considerably thinner than that of the L1recording film 32.

Here follows a description of the method of manufacturing an opticalrecording medium 10 according to this preferred embodiment.

FIGS. 2 to 5 are step drawings illustrating the method of manufacturingthe optical recording medium 10.

First, as shown in FIG. 2, a stamper 40 is used to perform injectionmolding of a substrate 11 having grooves 11 a and lands 11 b. Next, asshown in FIG. 5, the sputtering method is used to form, upon nearly theentire surface of the side of the substrate 11 on which the grooves 11 aand the lands 11 b are formed, a reflective film 34, a fourth dielectricfilm 33, an L1 recording film 32 and a third dielectric film 34 in thisorder, thereby forming an L1 layer 30. Here, the phase of the L1recording film 32 is normally in an amorphous phase immediately afterthe sputtering is completed.

Next, as shown in FIG. 4, ultraviolet curable acrylic resin isspin-coated onto the L1 layer 30, and by shining an ultraviolet raythrough a stamper 41 in the state with its surface covered with thestamper 41, an intermediate layer 12 having grooves 12 a and lands 12 bis formed. Next, as shown in FIG. 7, the sputtering method is used toform, upon nearly the entire surface of the intermediate layer 12 onwhich the grooves 11 a and the lands 11 b are formed, a seconddielectric film 23, an L0 recording film 22 and a first dielectric film21 in this order. Thus, an L0 layer 20 is completed. Here, the phase ofthe L0 recording film 22 is normally in an amorphous phase immediatelyafter the sputtering is completed.

Moreover, as shown in FIG. 1, ultraviolet curable acrylic resin isspin-coated onto the L0 layer 20, and by shining an ultraviolet ray, alight transmission layer 13 is formed. This completes all filmdeposition steps. In this specification, the optical recording medium inthe state with the film deposition steps complete may also be called the“optical recording medium precursor.”

Next, the optical recording medium precursor is placed upon the rotarytable of a laser irradiation apparatus (not shown) and rotated whilebeing continuously irradiated with a rectangular laser beam having ashorter length in the direction along the track and a longer length inthe direction perpendicular to the track. By shifting the irradiationposition in the direction perpendicular to the track each time theoptical recording medium precursor makes one revolution, the rectangularlaser beam can be shined over nearly the entire surface of the L0recording film 22 and the L1 recording film 32. Thereby, the phasechange material making up the L0 recording film 22 and the L1 recordingfilm 32 is heated to a temperature equal to or higher than thecrystallization temperature thereof and then cooled slowly, so theentire surface of the L0 recording film 22 and the L1 recording film 32is put into the crystalline state, namely the unrecorded state. Thisprocess is called “an initializing process” in this specification.

When the initializing process is completed, the optical recording medium10 is competed.

As described above, it is possible to record the desired digital dataonto an optical recording medium 10 thus manufactured by aligning thefocus of the laser beam during recording to either the L0 recording film22 or the L1 recording film 32 to form recording marks. In addition,when data is recorded onto the L0 recording film 22 and/or L1 recordingfilm 32 of the optical recording medium 10 in this manner, as describedabove, by aligning the focus of a laser beam set to playback power toeither the L0 recording film 22 or the L1 recording film 32 anddetecting the amount of light reflected, it is possible to play back thedigital data thus recorded.

Next, a pulse train pattern used for recording data in the L0 recordingfilm 22 and a pulse train pattern used for recording data in the L1recording film 32 will be described in detail.

FIG. 6 are a set of waveform diagrams showing pulse train patterns usedfor recording data in the L0 recording film 22 and the L1 recording filmwherein FIG. 6(a) shows a case of recording a 2T signal, FIG. 6(b) showsa case of recording a 3T signal, FIG. 6(c) shows a case of recording a4T signal and FIG. 6(d) shows a case of recording one of a 5T signal toan 8T signal.

As shown in FIGS. 6(a) to (d), in this embodiment, when data are to berecorded in the L0 recording film 22 or the L1 recording film 32, thepower of a laser beam is modulated between three levels (three values)of a recording power (Pw), an erasing power (Pe) and a bottom power(Pb). The level of the recording power (Pw) is set to such a high levelthat the L0 recording film 22 or the L1 recording film 32 can be meltedby the irradiation with the laser beam and is set to Pw0 when data areto be recorded in the L0 recording film 22 and Pw1 when data are to berecorded in the L1 recording film 32. The level of the erasing power(Pe) is set to such a level that the L0 recording film 22 or the L1recording film 32 can be heated to a temperature equal to or higher thana crystallization temperature thereof and is set to Pe0 with respect todata recorded in the L0 recording film 22 and Pe1 with respect to datarecorded in the L1 recording film 32. The level of the bottom power (Pb)is set to such a low level that the melted L0 recording film 22 or L1recording film 32 can be cooled even if it is irradiated with the laserbeam and is set to Pb0 when data are to be recorded in the L0 recordingfilm 22 and Pb1 when data are to be recorded in the L1 recording film32.

The details of the recording power (Pw0, Pw1), the erasing power (Pe0,Pe1) and the bottom power (Pb0, Pb1) will be described later.Hereinafter, a reference to simply the recording power (Pw), erasingpower (Pe) or bottom power (Pb) means the recording power (Pw0), erasingpower (Pe0) or bottom power (Pb0) when data are to be recorded in the L0recording film 22 and means the recording power (Pw1), erasing power(Pe1) or bottom power (Pb1) when data are to be recorded in the L1recording film 32.

First, as shown in FIG. 6(a), in the case of recording a 2T signal inthe L0 recording film 22 or the L1 recording film 32, the number ofpulses is set to 1 and a cooling interval T_(cl) is inserted thereafter.Here, the number of pulses is defined by the number of times the powerof the laser beam is raised to the recording power (Pw). Further, inthis specification, a first pulse is defined as a top pulse, a finalpulse is defined as a last pulse and any pulse present between the toppulse and the last pulse is defined as a multi-pulse. However, in thecase where the number of pulses is set to 1 as shown in FIG. 6(a), thepulse is the top pulse.

At the cooling interval T_(cl), the power of a laser beam is set to thebottom power (Pb). In this manner, in this specification, a lastinterval during which the power of a laser beam is set to the bottompower (Pb) is defined as the cooling interval. Therefore, in the case ofrecording a 2T signal, the power of the laser beam is set to the erasingpower (Pe) before the time t₁₁ set to the recording power (Pw) duringthe period (T_(top)) from the time t₁₁ to the time t₁₂, set to thebottom power (Pb) during the period (T_(cl)) from the time t₁₂ to thetime t₁₃ and set to the erasing power (Pe) after the time t₁₃.

Further, as shown in FIG. 6(b), in the case of recording a 3T signal inthe L0 recording film 22 or the L1 recording film 32, the number ofpulses is set to 2 and a cooling interval is inserted thereafter.Therefore, in the case of recording a 3T signal, the power of the laserbeam is set to the erasing power (Pe) before the time t₂₁, set to therecording power (Pw) during the period (T_(top)) from the time t₂, tothe time t₂₂ and the period (T_(lp)) from the time t₂₃ to the time t₂₄,set to the bottom power (Pb) during the period (T_(off)) from the timet₂₂ to the time t₂₃ and the period (T_(cl)) from the time t₂₄ to thetime t₂₅ and set to the erasing power (Pe) after the time t₂₅.

Furthermore, as shown in FIG. 6(c), in the case of recording a 4T signalin the L0 recording film 22 or the L1 recording film 32, the number ofpulses is set to 3 and a cooling interval is inserted thereafter.Therefore, in the case of recording a 4T signal, the power of the laserbeam is set to the erasing power (Pe) before the time t₃₁, set to therecording power (Pw) during the period (T_(top)) from the time t₃, tothe time t₃₂, the period (T_(mp)) from the time t₃₃ to the time t₃₄ andthe period (T_(lp)) from the time t₃₅ to the time t₃₆, set to the bottompower (Pb) during the period (T_(off)) from the time t₃₂ to the timet₃₃, the period (T_(off)) from the time t₃₄ to the time t₃₅ and theperiod (T_(cl)) from the time t₃₆ to the time t₃₇ and set to the erasingpower (Pe) after the time t₃₇.

In addition, as shown in FIG. 6(d), in the case of recording any one ofa 5T signal to an 8T signal in the L0 recording film 22 or the L1recording film 32, the number of pulses is correspondingly set to one of4 to 7 and a cooling interval T_(cl) is inserted thereafter. Therefore,the number of multi-pulses is set to 2 to 5 correspondingly to a 5Tsignal to an 8T signal. In this case, the power of the laser beam is setto the recording power (Pw) during the period T_(top) from the time t₄₁to the time t₄₂, the periods T_(mp) corresponding to those from the timet₄₃ to the time t₄₄, from the time t₄₅ to the time t₄₆ and the like andthe period T_(lp) from the time t₄₇ to the time t₄₈, set to the bottompower (Pb) during the off periods T_(off) corresponding to those fromthe time t₄₂ to the time t₄₃, from the time t₄₆ to the time t₄₇ and thelike and the cooling interval T_(cl) from the time t₄₈ to the time t₄₉,and set to the erasing power (Pe) during the other periods.

As a result, at a region where one of recording signals among a 2Tsignal to a 8T signal is to be recorded, the L0 recording film 22 or theL1 recording film 32 melted by the irradiation with the laser beam ofthe recording power (Pw1) is quickly cooled during the cooling intervalT_(cl) and the phase thereof is changed to the amorphous phase. On theother hand, at the other regions, the L0 recording film 22 or the L1recording film 32 is heated to a temperature equal to or higher than thecrystallization temperature thereof and gradually cooled as the laserbeam moves away, thereby being crystallized.

Here, in the case of recording data in the L0 recording film 22, each ofthe pulse width T_(top) of the top pulse, the pulse width T_(mp) of eachof the multi-pulses, the pulse width T_(lp) of the last pulse and thecooling interval T_(cl) is set to be constant when any one of a 2Tsignal to a 8T signal is to be recorded. Hereinafter, the pulse width ofthe top pulse, the pulse width of each of the multi-pulses, the pulsewidth of the last pulse and the cooling interval in the case ofrecording data in the L0 recording film 22 are referred to as T_(top0),T_(mp0), T_(lp0) and T_(cl0). Similarly, in the case of recording datain the L1 recording film 32, each of the pulse width T_(top) of the toppulse, the pulse width T_(mp) of each of the multi-pulses, the pulsewidth T_(lp) of the last pulse and the cooling interval T_(cl) is set tobe constant when any one of a 2T signal to a 8T signal is to be recordedand. hereinafter, the pulse width of the top pulse, the pulse width ofeach of the multi-pulses, the pulse width of the last pulse and thecooling interval in the case of recording data in the L0 recording film22 are referred to as T_(top1), T_(mp1), T_(lp1), and T_(cl1). However,reference to simply T_(top), T_(mp), T_(lp) or T_(cl1), means T_(top0),T_(mp0), T_(lpo) or T_(cl0) when data are to be recorded in the L0recording film 22 and means T_(top1), T_(mp1), T_(lp1) or T_(cl1) whendata are to be recorded in the L1 recording film 32.

As described above, the L0 layer 20 is provided with no reflective layeror only a very thin reflective film (3 to 15 nm) even if it includes areflective film, while the L1 layer 30 is provided with the reflectivefilm 34 having a thickness of 20 to 200 nm. Therefore, the heatradiation characteristic of the L0 layer 20 is lower than that of the L1layer 30 and the phase change material contained in the L0 layer 20tends to be re-crystallized.

In view of this, in this embodiment, in order to prevent the phasechange material contained in the L0 layer 20 from being re-crystallized,the ratio (Pe0/Pw0) of the recording power (Pw0) and the erasing power(Pe0) when data are to be recorded in the L0 recording film 22 is set tobe smaller than the ratio (Pe1/Pw1) of the recording power (Pw1) and theerasing power (Pe1) when data are to be recorded in the L1 recordingfilm 32, thereby reducing thermal interference in the L0 layer 20 inwhich only a low cooling effect is obtained to suppress there-crystallization of the phase change material contained in the L0layer 20. In this case, if the ratio (Pe0/Pw0) is set too low withrespect to the ratio (Pe1/Pw1), the erasing efficiency in the L0 layer20 becomes low. On the other hand, if the ratio (Pe0/Pw0) is set to betoo close to the ratio (Pe1/Pw1), thermal interference cannot be reducedin a desired manner and it is difficult to prevent the phase changematerial contained in the L0 layer 20 from being re-crystallized.Therefore, it is preferable to set the ratio (Pe0/Pw0) to be about 0.38to 0.66 times the ratio (Pe1/Pw1), is more preferable to set it to beabout 0.44 to 0.55 times the ratio (Pe1/Pw1) and is particularlypreferable to set it to be about 0.50 times the ratio (Pe1/Pw1). If therelationship between the ratio (Pe0/Pw0) and the ratio (Pe1/Pw1) is setin this manner, the erasing efficiency can be maintained within asuitable range for practical use and thermal interference can beeffectively reduced.

However, when data are to be recorded in the L1 recording film 32, sincethe laser beam is projected onto the L1 recording film 32 via the L0layer 20, the laser beam has been considerably attenuated when itreaches the L1 recording film 32. Therefore, in order to sufficientlymelt the L1 recording film 32, it is necessary to set the recordingpower (Pw1) to be higher than the recording power (Pw0) used forrecording data in the L0 recording film 22, namely, Pw0<Pw1.

Further, in this embodiment, in order to prevent the phase changematerial contained in the L0 layer 20 from being re-crystallized, thepulse width T_(top0) of the top pulse when data are to be recorded inthe L0 layer 20 is set to be shorter than the pulse width T_(top1) ofthe top pulse when data are to be recorded in the L1 layer 30, therebyfurther reducing thermal interference in the L0 layer 20 in which only alow cooling effect is obtained to more effectively suppress there-crystallization of the phase change material contained in the L0layer 20. In this case, if the pulse width T_(top0) is set too shortwith respect to the pulse width T_(top1), there is a risk of the L0recording film 22 not being heated to a temperature equal to or higherthan the melting point thereof. On the other hand, if the pulse widthT_(top0) is set close to the pulse width T_(top1), thermal interferencecannot be reduced in a desired manner and it is difficult to prevent thephase change material contained in the L0 layer 20 from beingre-crystallized. Therefore, it is preferable to set the pulse widthT_(top0) to be about 0.40 to 0.75 times the pulse width T_(top1), ismore preferable to set it to be about 0.49 to 0.55 times the pulse widthT_(top1) and is particularly preferable to set it to be about 0.55 timesthe pulse width T_(top1). If the relationship between the pulse widthT_(top0) and the pulse width T_(top1) is set in this manner, it ispossible to reliably heat the L0 recording film 22 to a temperatureequal to or higher than the melting point thereof and thermalinterference can be effectively reduced. Here, the pulse widths ofT_(lp0) and T_(lp1) can be set to be equal to the pulse widths T_(top0)and T_(top0).

Furthermore, in this embodiment, in order to prevent the phase changematerial contained in the L0 layer 20 from being re-crystallized, thepulse width T_(mp0) of each of the multi-pulses when data are to berecorded in the L0 layer 20 is set to be shorter than the pulse widthT_(mp1) of each of the multi-pulses when data are to be recorded in theL1 layer 30, thereby further reducing thermal interference in the L0layer 20 in which only a low cooling effect is obtained to moreeffectively suppress the re-crystallization of the phase change materialcontained in the L0 layer 20. In this case, if the pulse width T_(mp0)is set too short with respect to the pulse width T_(mp1), there is arisk of the L0 recording film 22 not being heated to a temperature equalto or higher than the melting point thereof. On the other hand, if thepulse width T_(mp0) is set close to the pulse width T_(mp1), thermalinterference cannot be reduced in a desired manner and it is difficultto prevent the phase change material contained in the L0 layer 20 frombeing re-crystallized. Therefore, it is preferable to set the pulsewidth T_(top0) to be about 0.48 to 0.58 times the pulse width T_(mp1),is more preferable to set it to be about 0.50 to 0.53 times the pulsewidth T_(mp1) and is particularly preferable to set it to be about 0.50times the pulse width T_(mpl). If the relationship between the pulsewidth T_(mp0) and the pulse width Tmp₁ is set in this manner, it ispossible to reliably heat the L0 recording film 22 to a temperatureequal to or higher than the melting point thereof and thermalinterference can be effectively reduced.

Moreover, in this embodiment, in order to prevent the phase changematerial contained in the L0 layer 20 from being re-crystallized, thecooling interval T_(cl0) when data are to be recorded in the L0 layer 20is set to be longer than the cooling interval T_(cl1) when data are tobe recorded in the L1 layer 30, thereby further reducing thermalinterference in the L0 layer 20 in which only a low cooling effect isobtained to more effectively suppress the re-crystallization of thephase change material contained in the L0 layer 20. In this case, if thecooling interval T_(cl0) is set too long with respect to the coolinginterval T_(cl1), the erasing efficiency in the L0 recording film 22becomes low. On the other hand, if the cooling interval T_(cl0) is setclose to the cooling interval T_(cl1), thermal interference cannot bereduced in a desired manner and it is difficult to prevent the phasechange material contained in the L0 layer 20 from being re-crystallized.Therefore, it is preferable to set the cooling interval T_(cl0) to beabout 1.25 to 2.00 times the cooling interval T_(cl1), is morepreferable to set it to be about 1.25 to 1.50 times the cooling intervalT_(cl1) and is particularly preferable to set it to be about 1.25 timesthe cooling interval T_(cl1). If the relationship between the coolinginterval T_(cl0) and the cooling interval T_(cl1) is set in this manner,the erasing efficiency can be maintained within a suitable range forpractical use and thermal interference can be effectively reduced.

It is preferable to store “recording condition setting information” inthe optical recording medium 10 as information for identifying the pulsetrain patterns for the L0 recording film 22 and the L1 recording film32. If such recording condition setting information is stored in theoptical recording medium 10, the recording condition setting informationis read by an information recording apparatus when data are actuallyrecorded in the optical recording medium 10 by the user, and the pulsetrain patterns can be determined based on the thus read recordingcondition setting information. Therefore, for example, when the userrequests recording of data in the L0 layer 20, the information recordingapparatus sets the recording power, the erasing power and the bottompower to Pw0, Pe0 and Pb0, respectively, sets the pulse width of the toppulse, the pulse width of each of the multi-pulses, the pulse width ofthe last pulse and the cooling interval to T_(top0), T_(mp0), T_(lp0)and T_(cl0), respectively, and records data in the L0 recording film 22.On the other hand, when the user requests recording of data in the L1layer 30, the information recording apparatus sets the recording power,the erasing power and the bottom power to Pw1, Pe1 and Pb1,respectively, sets the pulse width of the top pulse, the pulse width ofeach of the multi-pulses, the pulse width of the last pulse and thecooling interval to T_(top1), T_(mp1), T_(lp1) and T_(cl1),respectively, and records data in the L1 recording film 32.

It is more preferable for the recording condition setting information toinclude not only information required for identifying the pulse trainpatterns for the L0 recording film 22 and the L1 recording film 32 butalso information required for identifying various conditions such as thelinear recording velocity required to record data in the opticalrecording medium 10. The recording condition setting information may berecorded in the optical recording medium 10 as a wobble signal orpre-pits, or it may be recorded as data in the L0 recording film 22and/or the L1 recording film 32. Further, the recording conditionsetting information may include not only information directly indicatingvarious conditions required to record data but also information capableof indirectly identifying the pulse train patterns by specifying any ofvarious conditions stored in the information recording apparatus inadvance.

FIG. 7 is a schematic drawing of the major components of an informationrecording apparatus 50 for recording data in the optical recordingmedium 10.

As shown in FIG. 7, the information recording apparatus 50 is equippedwith a spindle motor 52 for rotating an optical recording medium 10, anoptical head 53 for shining a laser beam onto the optical recordingmedium 10, a controller 54 for controlling the operation of the spindlemotor 52 and the optical head 53, a laser driving circuit 55 thatsupplies a laser driving signal to the optical head 53, and a lensdriving circuit 56 that supplies a lens driving signal to the opticalhead 53.

Moreover, as shown in FIG. 7, the controller 54 includes a focusingservo circuit 57, a tracking servo circuit 58, and a laser controlcircuit 59. When the focusing servo circuit 57 is activated, the focusis aligned with the recording surface of the rotating optical recordingmedium 10, and when the tracking servo circuit 58 is activated, the spotof the laser beam begins to automatically track the eccentric signaltrack of the optical recording medium 10. The focusing servo circuit 57and tracking servo circuit 58 are provided with an auto gain controlfunction for automatically adjusting the focusing gain and an auto gaincontrol function for automatically adjusting the tracking gain,respectively. In addition, the laser control circuit 59 is a circuitthat generates the laser driving signal supplied by the laser drivingcircuit 55 and generates a laser driving signal based on recordingcondition setting information recorded on the optical recording medium10.

Note that the focusing servo circuit 57, tracking servo circuit 58 andlaser control circuit 59 need not be circuits incorporated in thecontroller 54 but can instead be components separate of the controller54. Moreover, they need not be physical circuits but can instead beaccomplished by software programs executed in the controller 54.

In the case of recording data in the optical recording medium 10 usingthe thus constituted information recording apparatus 50, as describedabove, the recording condition setting information recorded in theoptical recording medium 10 is read and pulse train patterns aredetermined based on the thus read recording condition settinginformation. Therefore, in the case of recording data in the L0 layer20, the information recording apparatus 50 uses the thus read recordingcondition setting information to set the recording power, the erasingpower and the bottom power to Pw0, Pe0 and Pb0, respectively and set thepulse width of the top pulse, the pulse width of each of themulti-pulses, the pulse width of the last pulse and the cooling intervalto T_(top0), T_(mp0), T_(lp0) and T_(cl0), respectively, and thenrecords data in the L0 recording film 22. On the other hand, in the caseof recording data in the L1 layer 30, the information recordingapparatus 50 uses the thus read recording condition setting informationto set the recording power, the erasing power and the bottom power toPw1, Pe1 and Pb1, respectively, and set the pulse width of the toppulse, the pulse width of each of the multi-pulses, the pulse width ofthe last pulse and the cooling interval to T_(top1), T_(mp1), T_(lp1)and T_(cl1), respectively, and then records data in the L1 recordingfilm 32.

As described above, in this embodiment, in the case of recording data inthe L0 layer 20 close to the light incidence plane 13 a, since the ratio(Pe0/Pw0) of the recording power (Pw0) and the erasing power (Pe0) isset to be smaller than the ratio (Pe1/Pw1) of the recording power (Pw1)and the erasing power (Pe1) in the case of recording data in the L1layer 30 far from the light incidence plane 13 a, it is possible tofurther reduce thermal interference in the L0 layer 20 in which only alow cooling effect is obtained and it is possible to suppress there-crystallization of the phase change material contained in the L0layer 20.

Further, in this embodiment, in the case of recording data in the L0layer 20 close to the light incidence plane 13 a, since the pulse widthT_(top0) of the top pulse, the pulse width T_(mp0) of each of themulti-pulses and the pulse width T_(lp0) of the last pulse are set to beshorter than the pulse width T_(top1) of the top pulse, the pulse widthT_(mp1) of each of the multi-pulses and the pulse width T_(lp1) of thelast pulse in the case of recording data in the L1 layer 30 far from thelight incidence plane 13 a and the cooling interval T_(cl0) is set to belonger than cooling interval T_(cl1) in the case of recording data inthe L1 layer 30 far from the light incidence plane 13 a, it is possibleto further reduce thermal interference in the L0 layer 20 in which onlya low cooling effect is obtained and it is possible to suppress there-crystallization of the phase change material contained in the L0layer 20.

The present invention is in no way limited to the aforementionedembodiment, but rather various modifications are possible within thescope of the invention as recited in the claims, and these are naturallyincluded within the scope of the invention.

For example, in the preferred embodiment above, an optical recordingmedium having two recording layers was described, but the opticalrecording media to which the present invention applies are not limitedthereto so the present invention is also applicable to optical recordingmedia having three or more recording layers.

In this case, when data are to be recorded in one of informationrecording layers other than an information recording layer farthest fromthe light incidence plane 13 a, the ratio Pe/Pw can be set to be smallerthan that when data are to be recorded in the information recordinglayer farthest from the light incidence plane 13 a, for example, about0.38 to 0.66 times the latter, and it is preferable to set the ratioPe/Pw to decrease stepwise as the information recording layer in whichdata are to be recorded is closer to the light incidence plane 13 a.Further, when data are to be recorded in one of information recordinglayers other than an information recording layer farthest from the lightincidence plane 13 a, the pulse width T_(top) (and the pulse widthT_(lp)) can be set to be shorter than the pulse width T_(top) when dataare to be recorded in the information recording layer farthest from thelight incidence plane 13 a, for example, about 0.40 to 0.75 times thelatter. Furthermore, when data are to be recorded in one of informationrecording layers other than an information recording layer farthest fromthe light incidence plane 13 a, the pulse width T_(mp) can be set to beshorter than that when data are to be recorded in the informationrecording layer farthest from the light incidence plane 13 a, forexample, about 0.48 to 0.58 times the latter. Moreover, when data are tobe recorded in one of information recording layers other than aninformation recording layer farthest from the light incidence plane 13a, the cooling interval T_(cl) can be set to be longer than that whendata are to be recorded in the information recording layer farthest fromthe light incidence plane 13 a, for example, about 1.25 to 2.00 timesthe latter. It is preferable to set the pulse width T_(top) (and thepulse width T_(lp)) and the pulse width T_(mp) to decrease stepwise andset the cooling interval T_(cl) to increase stepwise, as the informationrecording layer in which data are to be recorded is closer to the lightincidence plane 13 a.

As described above, according to the present invention, recording markshaving good shapes can be formed even when information in any one of theinformation recording layers is directly overwritten.

Here, the influence of thermal interference becomes pronounced as thewavelength of the laser beam used for recording data is shorter and thenumerical aperture (NA) of the objective lens used for converging thelaser beam is larger. Therefore, the present invention is particularlyeffective in the case where the quotient (λ/NA) of the wavelength λ ofthe laser beam used for reproducing data divided by the numericalaperture (NA) of the objective lens used to focus the laser beam isequal to or shorter than 700 nm, for example, where the numericalaperture NA is 0.7 (particularly, roughly 0.85) and the wavelength A ofthe laser beam is about 200 to 450 nm.

WORKING EXAMPLE

Hereinafter, a Working Example will be described concretely.

Fabrication of an Optical Recording Medium 10

A stamper 40 shown in FIG. 2 was first used to perform injection moldingof polycarbonate, thereby fabricating a substrate 11 having grooves 11 awhose depth was 34 mm and whose pitch was 0.32 μm and a thickness of 1.1mm.

Then, the substrate 11 was set in a sputtering apparatus (not shown) andan Ag alloy, a mixture of ZnS and SiO₂ (mole ratio of 80:20), AgSbTeGeand a mixture of ZnS and SiO₂ (mole ratio of 80:20) were sputtered inthis order on nearly the entire surface of the side of the substrate 11on which the grooves 11 a and the lands 11 b were formed, therebyforming an L1 layer 30, namely, a reflective film 34 having a thicknessof 100 nm, a fourth dielectric film 33 having a thickness of 15 nm, anL1 recording film 32 having a thickness of 12 nm and a third dielectricfilm 31 having a thickness of 80 nm.

Next, the substrate 11 formed with the L1 layer was picked out from thesputtering apparatus and an ultraviolet ray curable resin was appliedonto the third dielectric film 31 using a spin coating process. Further,an ultraviolet ray was shined on the surface of the spin-coatedultraviolet ray curable resin through a stamper 41 in the state with itssurface covered with the stamper 41, thereby forming an intermediatelayer 12 having grooves 12 a whose depth was 34 mm and whose pitch was0.32 μm and a thickness of 20 μm.

Then, the substrate 11 formed with the L1 layer 30 and the intermediatelayer 12 was set in the sputtering apparatus and Al₂O₃, SbTe and amixture of ZnS and SiO₂ (mole ratio of 80:20) were sputtered in thisorder on nearly the entire surface of the side of the intermediate layer12 on which the grooves 12 a and the lands 12 b are formed, therebyforming an L0 layer 20, namely, a second dielectric film 23 having athickness of 70 nm, an L0 recording film 22 having a thickness of 8 nmand a first dielectric film 21 having a thickness of 60 nm.

Further, after the substrate 11 formed with the L1 layer 30, theintermediate layer 12 and the L0 layer 20 was picked out from thesputtering apparatus, an ultraviolet ray curable resin was applied ontothe first dielectric film 21 using a spin coating process and anultraviolet ray was shined on the spin-coated ultraviolet ray curableresin, thereby forming a light transmission layer 13 having a thicknessof 100 μm. Thus, an optical recording medium precursor was fabricated.

Next, the optical recording medium precursor was placed upon the rotarytable of a laser irradiation apparatus (not shown) and rotated whilebeing continuously irradiated with a rectangular laser beam having ashorter length in the direction along the track and a longer length inthe direction perpendicular to the track. The irradiation position wasshifted in the direction perpendicular to the track each time theoptical recording medium precursor made one revolution, therebycrystallizing substantially the entire surface of the L0 recording film22 and the L1 recording film 32. Thus, an optical recording medium 10 tobe used in this Working Example was completed.

Setting the Power of a Laser Beam

Data were recorded in the L0 layer 20 and the L1 layer 30 of the thusfabricated optical recording medium 10 while the recording power (Pw)and the erasing power (Pe) are varied and jitter of the thus formedrecording marks were measured. The jitter was calculated based on aformula: σ/Tw (%) where Tw was one clock period by measuring clockjitter using a time interval analyzer and obtaining the fluctuation σ ofthe reproduced signal. Random signals in the (1,7) RLL modulation schemewere recorded as data by setting a clock frequency to 65.7 MHz (T=15.2nsec) and a linear recording velocity to 5.7 m/sec. The wavelength ofthe laser beam used for recording data was 405 nm and the numericalaperture of an objective lens used for converging the laser beam was0.85.

Further, in the case of recording data in the L0 recording film 22,T_(top0), T_(mp0), T_(lp0) and T_(cl0) were set to 0.2T, 0.2T, 0.2T and1.0T, respectively, and in the case of recording data in the L1recording film 32, T_(top1), T_(mp1), T_(lp1) and T_(cl1) were set to0.4T, 0.4T, 0.5T and 0.8T, respectively.

The results of measurement of jitter of signals reproduced from the L0layer 20 are shown in Table 1. TABLE 1 Pw0 (mW) Pe0 (mW) Pe0/Pw0 jitter(%) 5.5 1.8 0.327 16.8 5.5 1.5 0.273 13.1 5.5 1.3 0.236 12.4 5.5 1.00.182 14.1 5.5 0.8 0.145 17.0 5.0 2.0 0.400 17.7 5.0 1.8 0.360 14.0 5.01.5 0.300 11.7 5.0 1.3 0.260 11.1 5.0 1.0 0.200 11.9 5.0 0.8 0.160 12.35.0 0.5 0.100 14.7 4.5 2.0 0.444 18.3 4.5 1.8 0.400 14.9 4.5 1.5 0.33311.4 4.5 1.3 0.289 11.7 4.5 1.0 0.222 13.1 4.5 0.8 0.178 15.4

As shown in Table 1, in the case of recording data in the L0 recordingfilm 22, good jitter characteristics could be obtained when therecording power (Pw0) was 4.5 to 5.0 mW and the erasing power (Pe0) was1.3 to 1.5 mW. Here, the values of the recording power (Pw0) and theerasing power (Pe0) were defined as those at the surface of the opticalrecording medium. The value of the bottom power (Pb0) referred to laterwas similarly defined. Therefore, it was found that in the case ofrecording data in the L0 recording film 22, good jitter characteristicscould be obtained when the ratio (Pe0/Pw0) of the recording power (Pw0)and the erasing power (Pe0) was about 0.26 to 0.33.

FIG. 8 is a graph showing the relationship between the ratio (Pe0/Pw0)of the recording power (Pw0) and the erasing power (Pe0), and jitter. Itcan be seen from FIG. 8 that good jitter characteristics could beobtained when the ratio (Pe0/Pw0) of the recording power (Pw0) and theerasing power (Pe0) was about 0.26 to 0.33. From FIG. 8 it was confirmedthat particularly good jitter characteristics could be obtained when theratio (Pe0/Pw0) of the recording power (Pw0) and the erasing power (Pe0)was about 0.30.

The results of measurement of jitter of signals reproduced from the L1layer 30 are shown in Table 2. TABLE 2 Pw1 (mW) Pe1 (mW) Pe1/Pw1 jitter(%) 10.0 7.0 0.700 11.6 10.0 6.5 0.650 10.1 10.0 6.0 6.000 9.7 10.0 5.50.550 10.2 10.0 5.0 0.500 10.8 10.0 4.5 0.450 11.1 10.0 4.0 0.400 11.210.0 3.8 0.380 11.6 10.0 3.5 0.350 12.6 10.0 3.2 0.320 14.2 9.5 7.00.737 13.9 9.5 6.5 0.684 11.0 9.5 6.0 0.632 10.2 9.5 5.5 0.579 10.1 9.55.0 0.526 10.9 9.5 4.5 0.474 11.2 9.5 4.0 0.421 11.6 9.5 3.8 0.400 12.19.5 3.5 0.368 13.7 9.5 3.2 0.337 14.9

As shown in Table 2, in the case of recording data in the L1 recordingfilm 32, good jitter characteristics could be obtained when therecording power (Pw1) was 9.5 to 10.0 mW and the erasing power (Pe1) was5.0 to 6.5 mW. Here, the values of the recording power (Pw1) and theerasing power (Pe1) were defined as those at the surface of the opticalrecording medium. The value of the bottom power (Pb1) referred to laterwas similarly defined. Therefore, it was found that in the case ofrecording data in the L1 recording film 32, good jitter characteristicscould be obtained when the ratio (Pe1/Pw1) of the recording power (Pw1)and the erasing power (Pe1) was about 0.50 to 0.68.

FIG. 9 is a graph showing the relationship between the ratio (Pe1/Pw1)of the recording power (Pw1) and the erasing power (Pe1), and jitter. Itcan be seen from FIG. 9 that good jitter characteristics could beobtained when the ratio (Pe1/Pw1) of the recording power (Pw1) and theerasing power (Pe1) was about 0.50 to 0.68. From FIG. 9 it was confirmedthat particularly good jitter characteristics could be obtained when theratio (Pe1/Pw1) of the recording power (Pw1) and the erasing power (Pe1)was about 0.60.

In view of the above, it was found that it was preferable to set theratio (Pe0/Pw0) to be about 0.38 to 0.66 times the ratio (Pe1/Pw1), wasmore preferable to set it to be about 0.44 to 0.55 times the ratio(Pe1/Pw1) and was particularly preferable to set it to be about 0.50times the ratio (Pe1/Pw1).

Setting Pulse Widths

Then, data were recorded in the L0 layer 20 and the L1 layer 30 of thethus fabricated optical recording medium 10 while the pulse widthT_(top) of the top pulse, the pulse width T_(mp) of each of themulti-pulses, the pulse width T_(lp) of the last pulse and the coolinginterval T_(cl) were varied and jitter of the thus formed recordingmarks were measured. In this Working Example, since the pulse widthT_(top) of the top pulse and the pulse width T_(lp) of the last pulsewere set to be equal to each other, the pulse width T_(top) of the toppulse will also be used to indicate the pulse width T_(lp) of the lastpulse below. Similarly to the above operation of setting the power ofthe laser beam, random signals in the (1,7) RLL modulation scheme wererecorded as data by setting a clock frequency to 65.7 MHz (T=15.2 nsec)and a linear recording velocity to 5.7 m/sec. The wavelength of thelaser beam used for recording data was 405 nm and the numerical apertureof the objective lens used for converging the laser beam was 0.85.

Further, when data were recorded in the L0 recording film 22, therecording power (Pw0), the erasing power (Pe0) and the bottom power(Pb0) were fixed to 5.0 mW, 1.5 mW and 0.1 mW, respectively, and whendata were recorded in the L1 recording film 32, the recording power(Pw1), the erasing power (Pe1) and the bottom power (Pb1) were fixed to10.0 mW, 6.0 mW and 0.1 mW, respectively.

First, data were recorded with the pulse width T_(top) of the top pulseand the pulse width T_(mp) of each of the multi-pulses set to be equalto each other and varying the widths thereof while fixing the coolinginterval T_(c,l), and jitter of the thus formed recording marks weremeasured.

The results of measurement of jitter of signals reproduced from the L0recording film 22 are shown in Table 3 and results of measurement ofjitter of signals reproduced from the L1 recording film 32 were shown inTable 4. TABLE 3 T_(top0) T_(mp0) T_(cl0) jitter(%) 0.16T 0.16T 1.0T17.1 0.18T 0.18T 1.0T 12.6 0.20T 0.20T 1.0T 10.9 0.22T 0.22T 1.0T 11.90.24T 0.24T 1.0T 13.5 0.26T 0.26T 1.0T 15.3

TABLE 4 T_(top1) T_(mp1) T_(cl1) jitter(%) 0.30T 0.30T 0.8T 14.1 0.35T0.35T 0.8T 12.3 0.38T 0.38T 0.8T 11.8 0.40T 0.40T 0.8T 10.9 0.42 0.420.8T 11.6 0.45T 0.45T 0.8T 12.7 0.50T 0.50T 0.8T 14.4

As shown in Table 3, in the case of recording data in the L0 recordingfilm 22, good jitter characteristics could be obtained when the pulsewidth T_(top0) of the top pulse and the pulse width T_(mp0) of each ofthe multi-pulses were 0.20T. Further, as shown in Table 4, in the caseof recording data in the L1 recording film 32, good jittercharacteristics could be obtained when the pulse width T_(top1) of thetop pulse and the pulse width T_(mp1) of each of the multi-pulses were0.40T.

Then, data were recorded by fixing one of the pulse width T_(top) of thetop pulse and the pulse width T_(mp) of each of the multi-pulses to thethus obtained pulse width, namely, 0.20T in the case of recording datain the L0 recording film 22 or 0.40T in the case of recording data inthe L1 recording film 32 and varying the other, and jitter of the thusformed recording marks were measured.

The results of measurement of jitter of signals reproduced from the L0recording film 22 are shown in Table 5 and results of measurement ofjitter of signals reproduced from the L1 recording film 32 are shown inTable 6. TABLE 5 T_(top0) T_(mp0) T_(cl0) jitter(%) 0.20T 0.18T 1.0T12.2 0.20T 0.20T 1.0T 10.9 0.20T 0.22T 1.0T 11.7 0.20T 0.24T 1.0T 13.10.20T 0.26T 1.0T 13.4 0.18T 0.20T 1.0T 11.6 0.22T 0.20T 1.0T 10.6 0.24T0.20T 1.0T 10.9 0.26T 0.20T 1.0T 11.2 0.30T 0.20T 1.0T 11.5 0.35T 0.20T1.0T 11.9 0.40T 0.20T 1.0T 12.4

TABLE 6 T_(top1) T_(mp1) T_(cl1) jitter(%) 0.40T 0.35T 0.8T 12.1 0.40T0.38T 0.8T 11.1 0.40T 0.40T 0.8T 10.9 0.40T 0.42T 0.8T 11.4 0.40T 0.45T0.8T 12.3 0.35T 0.40T 0.8T 11.7 0.38T 0.40T 0.8T 11.6 0.42T 0.40T 0.8T11.1 0.45T 0.40T 0.8T 11.4

As shown in Table 5, in the case of recording data in the L0 recordingfilm 22, if the pulse width T_(top0) of the top pulse was fixed to0.20T, good jitter characteristics could be obtained when the pulsewidth T_(mp0) of each of the multi-pulses was 0.20T to 0.22T and bestjitter characteristics could be obtained when the pulse width T_(mp0) ofeach of the multi-pulses was 0.20T. On the other hand, if the pulsewidth T_(mp0) of each of the multi-pulses was fixed to 0.20T, goodjitter characteristics could be obtained when the pulse width T_(top0)of the top pulse was 0.18T to 0.30T and best jitter characteristicscould be obtained when the pulse width T_(top0) of the top pulse was0.22T. Further, as shown in Table 6, in the case of recording data inthe L1 recording film 32, if the pulse width T_(top1) of the top pulsewas fixed to 0.40T, good jitter characteristics could be obtained whenthe pulse width T_(mp1) of each of the multi-pulses was 0.38T to 0.42Tand best jitter characteristics could be obtained when the pulse widthT_(mp1) of each of the multi-pulses was 0.40T. On the other hand, if thepulse width T_(mp1) of each of the multi-pulses was fixed to 0.40T, goodjitter characteristics could be obtained when the pulse width T_(top1)of the top pulse was 0.40T to 0.45T and best jitter characteristicscould be obtained when the pulse width T_(top1) of the top pulse was0.40T.

Then, data were recorded by fixing the pulse width T_(top) of the toppulse and the pulse width T_(mp) of each of the multi-pulses to the thusobtained pulse width, namely, 0.20T in the case of recording data in theL0 recording film 22 or 0.40T in the case of recording data in the L1recording film 32 and varying the cooling interval T_(cl), and jitter ofthe thus formed recording marks were measured.

The results of measurement of jitter of signals reproduced from the L0recording film 22 are shown in Table 7 and results of measurement ofjitter of signals reproduced from the L1 recording film 32 are shown inTable 8. TABLE 7 T_(top0) T_(mp0) T_(cl0) jitter(%) 0.22T 0.20T 0.8T12.3 0.22T 0.20T 1.0T 10.6 0.22T 0.20T 1.2T 11.3 0.22T 0.20T 1.5T 13.3

TABLE 8 T_(top1) T_(mp1) T_(cl1) jitter(%) 0.40T 0.40T 0.4T 13.2 0.40T0.40T 0.6T 11.7 0.40T 0.40T 0.8T 10.9 0.40T 0.40T 1.0T 12.4 0.40T 0.40T1.2T 13.1 0.40T 0.40T 1.4T 14.3 0.40T 0.40T 1.6T 16.4

As shown in Table 7, in the case of recording data in the L0 recordingfilm 22, good jitter characteristics could be obtained when the coolinginterval T_(cl0) was 1.0T to 1.2T and best jitter characteristics couldbe obtained when the cooling interval T_(cl0) was 1.0T. On the otherhand, as shown in Table 8, in the case of recording data in the L1recording film 32, good jitter characteristics could be obtained whenthe cooling interval T_(cl) was 0.6T to 0.8T and best jittercharacteristics could be obtained when the cooling interval T_(cl1) was0.8T.

In view of the above, it was found that in the case of recording data inthe L0 recording film 22, good jitter characteristics could be obtainedby setting the pulse width T_(top0) of the top pulse (=T_(lp0)) to 0.18Tto 0.30T, particularly to 0.22T, the pulse width T_(mp0) of each of themulti-pulses to 0.20T to 0.22T, particularly to 0.20T, and the coolinginterval T_(cl0) to 1.0T to 1.2T, particularly to 1.0T. On the otherhand, it was found that in the case of recording data in the L1recording film 32, good jitter characteristics could be obtained bysetting the pulse width T_(top1) of the top pulse (=T_(lp1)) to 0.40T to0.45T, particularly to 0.40T, the pulse width T_(mp1) of each of themulti-pulses to 0.38T to 0.42T, particularly to 0.40T, and the coolinginterval T_(cl1) to 0.6T to 0.8T, particularly to 0.8T.

As described above, it was found that it was preferable to set the pulsewidth T_(top0) (and T_(lp0)) to be about 0.40 to 0.75 times the pulsewidth T_(top1) (and T_(lp1)), was more preferable to set it to be about0.49 to 0.55 times the pulse width T_(top1) (and T_(lp)) and wasparticularly preferable to set it to be about times the pulse widthT_(top1) (and T_(lp1)). Further, it was found that it was preferable toset the pulse width T_(mp0) to be about 0.48 to 0.58 times the pulsewidth T_(mp), was more preferable to set it to about 0.50 to 0.53 timesthe pulse width T_(mp0) and was particularly preferable to set it to beabout 0.50 time the pulse width T_(mp1). Moreover, it was found that itwas preferable to set the cooling interval T_(cl0) to be about 1.25 to2.00 times the cooling interval T_(cl1), was more preferable to set itto be about 1.25 to 1.50 times the cooling interval T_(cl1) and wasparticularly preferable to set it to be about 1.25 times the coolinginterval T_(cl1).

1. An information recording method for recording information in a data rewritable type optical recording medium having at least stacked first and second information recording layers by projecting a laser beam thereonto whose power is modulated between a plurality of powers including at least a recording power (Pw) and an erasing power (Pe) via a light incidence plane, the information recording method comprising steps of setting λ/NA to be equal to or shorter than 700 nm where λ is a wavelength of the laser beam and NA is a numerical aperture (NA) of an objective lens, and, when recording information in the optical recording medium, setting a ratio (Pe/Pw) of the recording power and the erasing power when information is to be recorded in the first information recording layer to be smaller than that when information is to be recorded in the second information recording layer.
 2. An information recording method in accordance with claim 1, wherein the first information recording layer is located on the side of the light incidence plane with respect to the second information recording layer.
 3. An information recording method in accordance with claim 1, wherein information is recorded with at least one of a pulse width of a top pulse and a pulse width of a multi-pulse when information is to be recorded in the first information recording layer set to be narrower than a corresponding pulse width(s) when information is to be recorded in the second information recording layer.
 4. An information recording method in accordance with claim 1, wherein information is recorded with a cooling interval set to be longer when information is to be recorded in the first information recording layer than when information is to be recorded in the second information recording layer.
 5. An information recording method in accordance with claim 1, wherein the laser beam has a wavelength of 200 to 450 nm.
 6. An information recording method for recording information in a data rewritable type optical recording medium having at least stacked first and second information recording layers by projecting a laser beam thereonto whose power is modulated between a plurality of powers including at least a recording power (Pw) and an erasing power (Pe) via a light incidence plane, the information recording method comprising a step of, when recording information in the optical recording medium, setting a ratio (Pe/Pw) of the recording power and the erasing power when information is to be recorded in the first information recording layer to be 0.38 to 0.66 times that when information is to be recorded in the second information recording layer.
 7. An information recording method in accordance with claim 6, wherein the first information recording layer is located on the side of the light incidence plane with respect to the second information recording layer.
 8. An information recording method in accordance with claim 6, wherein information is recorded with at least one of a pulse width of a top pulse and a pulse width of a multi-pulse when information is to be recorded in the first information recording layer set to be narrower than a corresponding pulse width(s) when information is to be recorded in the second information recording layer.
 9. An information recording method in accordance with claim 6, wherein information is recorded with a cooling interval set to be longer when information is to be recorded in the first information recording layer than when information is to be recorded in the second information recording layer.
 10. An information recording method in accordance with claim 6, wherein information is recorded with λ/NA set to be equal to or shorter than 700 nm, where λ is a wavelength of the laser beam and NA is a numerical aperture (NA) of an objective lens.
 11. An information recording method in accordance with claim 10, wherein the laser beam has a wavelength of 200 to 450 nm.
 12. An information recording apparatus for recording information in a data rewritable type optical recording medium having at least stacked first and second information recording layers by projecting a laser beam thereonto whose power is modulated between a plurality of powers including at least a recording power (Pw) and an erasing power (Pe) via a light incidence plane, the information recording apparatus being constituted so as to set λ/NA to be equal to or shorter than 700 nm where λ is a wavelength of the laser beam and NA is a numerical aperture (NA) of an objective lens, and, when recording information in the optical recording medium, set a ratio (Pe/Pw) of the recording power and the erasing power when information is to be recorded in the first information recording layer to be smaller than that when information is to be recorded in the second information recording layer.
 13. An information recording apparatus in accordance with claim 12, wherein the first information recording layer is located on the side of the light incidence plane with respect to the second information recording layer.
 14. An information recording apparatus in accordance with claim 12, wherein the laser beam has a wavelength of 200 to 450 nm.
 15. An information recording apparatus in accordance with claim 12, wherein information is recorded with a ratio (Pe/Pw) of the recording power and the erasing power when information is to be recorded in the first information recording layer set to be 0.38 to 0.66 times that when information is to be recorded in the second information recording layer.
 16. An optical recording medium which has at least stacked first and second information recording layers and in which information can be recorded by projecting a laser beam thereonto whose power is modulated between a plurality of powers including at least a recording power (Pw) and an erasing power (Pe) via a light incidence plane, the optical recording medium comprising setting information required for setting λ/NA to be equal to or shorter than 700 nm where λ is a wavelength of the laser beam and NA is a numerical aperture (NA) of an objective lens, and, when recording information therein, setting a ratio (Pe/Pw) of the recording power and the erasing power when information is to be recorded in the first information recording layer to be smaller than that when information is to be recorded in the second information recording layer.
 17. An optical recording medium in accordance with claim 16, wherein the first information recording layer is located on the side of the light incidence plane with respect to the second information recording layer.
 18. An optical recording medium in accordance with claim 16, which further comprises setting information required for setting a ratio (Pe/Pw) of the recording power and the erasing power when information is to be recorded in the first information recording layer to be 0.38 to 0.66 times that when information is to be recorded in the second information recording layer.
 19. An optical recording medium in accordance with claim 16, which further comprises a light transmission layer for forming an optical path of the laser beam and the light transmission layer has a thickness of 30 to 200 μm. 